Due to the continuous growth in data traffic and bandwidth requirements, faster optical interconnects are urgently needed. This—in turn—entails an increasing demand of low-power high-speed optical transceivers. In order to meet this demand, a shift towards advanced modulation formats, such as multi-level signaling, seems promising. For example, a 4-level pulse-amplitude modulation (PAM4) signal needs half of the bandwidth of a typical non-return-to-zero (NRZ) signal.
However, multi-level signaling is known to be more sensitive to the linearity of components. In an optical system, the electro-optical modulator is usually the most non-linear element, as its transfer function is typically strongly bias- and voltage-dependent, so that the generation of a clean PAM4 eye requires, often, a power-hungry electrical digital-to-analog converter (DAC) or pre-distortion of the electrical signal to produce equidistant optical levels.
In order to alleviate these issues, the modulators can be arranged in a 2-bit optical DAC configuration, as described in a paper by W. Huang et al. entitled “Optical DAC for Generation of PAM4 Using Parallel Electro-Absorption Modulators” ECOC 2016; 42nd European Conference on Optical Communication, Dusseldorf, Germany, 2016, pp. 1-3). This approach, where each modulator is employed to create a binary NRZ signal, permits to bypass the non-linearity of the modulator, thus removing the linearity constraints from the transmitter side—in both the electrical and optical domain—and enabling the use of simpler low-power NRZ driver topologies.
A similar approach can be used in a variety of advanced modulation formats other than PAM4, as shown in U.S. Pat. No. 8,320,720 B2.